A band pass cavity filter preferably passes one narrow band of frequencies and attenuates all others with increasing attenuation above and below the pass frequency. Band pass filters are ideal when the interfering frequencies are not known to any degree of accuracy or when very broadband filtering is needed. Collectively, the frequencies that are allowed to pass are called the pass band.
Bandpass filters are well known in the art. An example of a bandpass cavity filter is shown in FIG. 1. Cavity filter 2 includes a hollow cylindrical body 3 which has a coarse tuning rod 4 and a fine tuning rod 5. Apertures 6 are arranged with screws 7 for securing loop assemblies 8 to the cavity filter. When properly installed, the loops of loop assemblies 8 are contained in the cavity filter, while connectors, jacks, or receptacles 9 protrude out from the cavity filter. Connectors 9 are generally bi-directional, and connect to coaxial cables for either receiving or transmitting signals to or from the filter.
However, communications equipment, receivers particularly, may require additional isolation at specific frequencies. One solution to solve this problem is to connect separate notch filters to the bandpass filter to provide isolation at each of the desired frequencies. However, this is not always ideal as it requires multiple filters to be used, which can be costly, and requires a substantial amount of additional space to install extra cavity filters.
Thus, what is needed is a device to be used with a bandpass cavity filter to provide isolation at desired frequencies without the need for multiple cavity filters. What is also needed is such a device that is tunable.